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UART (Universal Asynchronous Receiver-Transmitter) transmits data asynchronously using start and stop bits instead of a clock signal. The receiving UART reads bits at a specific frequency known as the baud rate.

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Key Parameters

  • Baud rate — Communication speed (bits per second)
  • Start/Stop bits — Frame delimiters
  • Parity bit — Optional error checking
  • Data bits — 5–9 bits per frame
  • Flow control — Hardware handshaking via CTS/RTS

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Transfer Modes by Subsystem

SubsystemTransfer ModeBaud RatesNotes
LinuxFIFO, CPU DMA300 bps – 4 MbpsDMA ideal for Bluetooth modules
Boot (UEFI)FIFO onlyUp to 1152005–8 bits per character
aDSPFIFO only115200, 230400, 460800, 921600, 1M, 3M, 6M5–8 bits per character

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Interface Components

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Device Tree Sources

PlatformFile
Dragonwing IQ-8275arch/arm64/boot/dts/qcom/sa8295p.dtsi

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APIs

SubsystemHeader
Linuxinclude/linux/tty.h
BootQcomPkg/Include/HSUart.h
aDSPadsp_proc/core/api/buses/uart.h

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Software Configuration

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Linux Device Tree Example

4-wire UART (with flow control): 
uart7: serial@99c000 {
    compatible = "qcom,geni-uart";
    reg = <0 0x0099c000 0 0x4000>;
    clocks = <&gcc GCC_QUPV3_WRAP0_S7_CLK>;
    clock-names = "se";
    pinctrl-names = "default";
    pinctrl-0 = <&qup_uart7_cts>, <&qup_uart7_rts>,
                <&qup_uart7_tx>, <&qup_uart7_rx>;
    interrupts = <GIC_SPI 608 IRQ_TYPE_LEVEL_HIGH>;
    power-domains = <&rpmhpd SA8295P_CX>;
    interconnect-names = "qup-core", "qup-config";
    status = "disabled";
};
QUPAC access control: 
// 4-wire HS UART
{ QUPV3_0_SE7, QUPV3_PROTOCOL_UART_4W, QUPV3_MODE_FIFO, AC_HLOS, TRUE, TRUE, FALSE }

// 2-wire UART (no flow control)
{ QUPV3_0_SE5, QUPV3_PROTOCOL_UART_2W, QUPV3_MODE_FIFO, AC_HLOS, TRUE, FALSE, FALSE }

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Kernel Configuration

CONFIG_QCOM_GENI_SE=y
CONFIG_SERIAL_QCOM_GENI=y
Enable the UART node in the device tree: 
+serial1 = &uart7;

+&uart7 {
+    status = "ok";
+};

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Verification

1

Verify UART device registration

ls /dev/ttyHS*
# Expected: /dev/ttyHS1

dmesg | grep ttyH
2

Configure UART settings

stty -F /dev/ttyHS1 115200
3

Run loopback test

Open two SSH terminals.Terminal 1 (write):
echo "Hello UART" > /dev/ttyHS1
Terminal 2 (read):
cat /dev/ttyHS1

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Debugging

mount -t debugfs none /sys/kernel/debug
echo -n "file qcom_geni_serial.c +p" > /sys/kernel/debug/dynamic_debug/control
echo -n "file serial_core.c +p" > /sys/kernel/debug/dynamic_debug/control
dmesg | grep -i uart
dmesg | grep ttyHS

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Troubleshooting

lsmod | grep qcom_geni
cat /proc/device-tree/soc/serial@*/status
ls -la /dev/tty*
  • Verify device tree status is "ok"
  • Confirm CONFIG_SERIAL_QCOM_GENI=y
stty -F /dev/ttyHS1 -a
stty -F /dev/ttyHS1 115200
stty -F /dev/ttyHS1 crtscts    # enable flow control
stty -F /dev/ttyHS1 -crtscts   # disable flow control

ls -la /dev/ttyHS*
usermod -a -G dialout $USER

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